The present invention relates generally to an improvement in ion sources available for producing a number of metal ions.
It is well known in the field of ion sources that electron bombardment ionization behaviors due to gaseous discharge are utilized in a wide range. This is generally termed "electron bombardment type." In this instance, in order to attain efficient utilization of the electron bombardment ionization phenomena, electrons are excited to follow spiral trajectories to increase effective electron path length by application of a magnetic field and an electric field in an orthogonal arrangement.
More specifically, in the conventional ion sources of the electron bombardment type, an ionizable metal material is heated within a metal vaporization furnace provided with a heater and a thermocouple therein. Then, the heated material in the vaporized form is supplied to a discharge chamber wherein voltage is supplied to a filament to produce electrons and also a different voltage is supplied to a cyclindrical anode placed around the filament to establish an electric field between the anode and the filament. Furthermore, a magnetic field perpendicular to the thus established electric field is induced so as to improve ionization efficiency by provision of a magnet coil, thereby permitting electrons to follow spiral trajectories with the resultant increase in electron path length. The provision of the magnet coil is accomplished by winding a fine wire outside the discharge chamber in a spiral configuration. Thereafter, an ion beam is led out through an opening of the discharge chamber in response to application of positive or negative high voltage to an extractor electrode assembly. The ion sources of the electron bombardment type briefly described above, however, have the disadvantages of being large and weighty because of the magnetic coil provided outside the discharge chamber. In addition, other various difficulties are encountered in attaining a magnetic field of high field strength: for example, heat rediation due to Joule heat from the magnetic coil, electric insulation required for high power supply, etc.
Furthermore, when it is desired to ionize metal materials of low vapor pressures and high melting points, the whole of the discharge chamber should be maintained at a predetermined high temperature, this being contrary to the requirement for shielding the magnetic coil against the heat radiation. Operation at high temperatures and high field strength magnetic field is not obtainable in the conventional ion sources employing the magnet coil.